Living cationic polymerization of vinyl ethers with carboxylic acid/tin tetrahalide initiating systems. II. Effects of steric hindrance of carboxylate counteranions

Effects of steric crowding of the substituent of carboxylate counteranions on living cationic polymerization of isobutyl vinyl ether (IBVE) were inves tigated with the use of two series of carboxylic acids with various carbony l substituents [RCOOH; R = (aliphatic series) CH,CH,, (CH,),CH, (CH,),CI(ar omatic series) C6H5CH2, (C6H5)(2)CH, (C6H5)(3)C] in conjunction with tin te trabromide (SnBr4) and 1,4-dioxane (DO) in toluene at 0 degrees C. The over all polymerization rate increased with increasing the bulkiness of the subs tituents R in both the series: R = CH, (1) = CH,CH, (1) < (CH3)(2)CH (1.76) < (CH3)(3)C (2.31); C6H5CH2 (0.84) < (C6H5)(2)CH (0.98) < (C6H5)(3)C (1.74 ); the values in the parentheses show the relative polymerization rate. In all the polymerizations, the number-average molecular weight (M-n) of the p olymers was directly proportional to monomer conversion and in good agreeme nt with the calculated values, assuming that one RCOOH molecule forms one p olymer chain. The living nature of these polymerizations was further confir med by a linear increase in M, of the polymers upon sequential addition of a fresh monomer feed to the almost completely polymerized reaction mixtures . In the polymerizations with sterically less hindered carboxylic acids [R = CH3CH2, (CH3)(2)CH, C6H5CH2, (C6H5)(2)CH], the molecular weight distribut ion (MWD) of the polymers was very narrow (M,IM, < 1.1) throughout the poly merizations. In contrast, with bulkier substituent-containing counterparts [R = (CH3)(3)C, (C6H5)(3)C], the polymerizations led to the polymers of rel atively broad MWD (M-w/M-n congruent to 1.5 at ca. 100% monomer conversion) . The bulky substituents such as (CH3)(3)C and (C6H5)(3)C may decrease the interconversion rate between a dormant and an active species and increase t he time-average concentration of the active growing species. The stereoregu larity of the obtained polymers was not changed much with the steric enviro nment of the counteranion (meso: 66-69%). (C) 1999 John Wiley & Sons, Inc.